LED light sources are characterized by a high efficiency, a long lifetime and a comparatively low sensitivity against shocks and vibrations. Therefore, LED light sources can be used in illumination systems in which incandescent lamps have often been used so far, in particular in lamps for general lighting or in car headlights. Due to their high brightness, LED light sources are also suited for use in traffic lamps or in light sources for projection systems.
LED light sources are often used in combination with one or more optical elements which reduce the divergence of the emitted light. For example, the document US 2004/0264185 A1 describes an LED light source that contains an optical concentrator to reduce the divergence of the emitted light.
The document WO 98/12757 A1 describes a method to produce white light by luminescence conversion. In this case, the LED contains at least one LED chip which emits blue and/or ultraviolet light. The emitted blue and/or ultraviolet light passes through a layer which contains luminescence conversion particles that convert at least a part of the emitted light to a complementary color with a longer wavelength, for example to yellow light. The blue and/or ultraviolet light and the yellow light intermix to white light.
Another known method to produce white light with LED chips is color mixing. In this case, the light emitted by a plurality of LED chips with different colors, for example red, green and blue light, is intermixed to produce white light. Both methods to produce white light by luminescence conversion or by color mixing are based on the intermixing of light with at least two different wavelengths. In LED illumination devices which use one of these methods to produce white light the problem may occur that the intermixing of different colors to white light is not perfect. In this case, the light distribution does not show homogenous white light but unwanted colors in at least some directions in which light is emitted. This problem particularly occurs when an optical element, e.g. a primary lens or an optical concentrator, is used in combination with the LED light source.